This invention is generally directed to improvements in television receivers, and specifically to AFC (automatic frequency control) filters for use in such receivers.
Television receivers typically include a tuner whose local oscillator develops a high frequency signal which is mixed with an incoming television signal to develop an IF (intermediate frequency) signal. Because it is important to carefully control the frequency of the local oscillator's signal, the receiver includes AFC circuitry which develops an error signal whose value is a function of any error in the local oscillator's frequency. That error signal is applied via an AFC filter to the local oscillator to adjust its frequency of oscillation such that the IF signal's picture carrier is at 45.75 megahertz.
Due to the way in which the error signal is ordinarily developed, it contains unwanted vertical and horizontal rate components and their harmonics which can frequency modulate the local oscillator. In the mixing process, both the 45.75 megahertz picture carrier and the 41.25 megahertz sound carrier take on the undesirable frequency modulation carried by the local oscillator. When such modulation occurs, the 45.75 megahertz picture carrier and the 41.25 megahertz sound carrier associated with the incoming signal become frequency modulated.
In so-called "inter-carrier" sound systems, the video and audio carriers are recovered from the IF signal by a common detector. Hence, most unwanted modulation components (due to the above-described modulation of the local oscillator) are somewhat mutually cancelling from both carriers. The unwanted modulation components which remain do not substantially effect faithful reproduction of monophonic sound.
In so-called "split sound" systems, the audio portion of the incoming signal is detected separately from the video portion. Hence, no cancelling of unwanted modulation components occurs, and those components are in the frequency spectrum which is reproduced. Consequently, "buzz" is developed in the reproduced audio.
To reduce the amount of buzz developed in a split sound system, the AFC filter requires a relatively long time constant in order to reduce the amplitude of the horizontal and vertical rate components in the error signal. However, the latter requirement conflicts with an equally important requirement that the AFC filter have a short time constant in order to cause the local oscillator to quickly lock to its required frequency. A solution to these conflicting requirements in the design of an AFC filter is provided by the present invention.